Five Illustrative Mental Disorders I 111
tion, for example, but would increase sevenfold among those over 85 (Evans et al., 1990). Past census projections have been particularly inaccurate regarding expansion of the oldest age groups, and the anticipated rise of AD over the next four decades is especially sensitive to such uncertainty.
International epidemiological studies have shown relatively little variation among populations in the incidence or prevalence of dementia, when adjusted for late age of onset (Rocca, Hofman, Brayne, Breteler, Clarke, Copeland et al., 1991). Earlier reports had suggested a low incidence and prevalence of AD among Asian populations. More systematic recent studies have found prevalence among Asians within the range of Western surveys (Zhang, Katzman, Kashani, Salmon, Jin, Cai et al., 1990), although the ratio of AD to other causes of dementia may differ between predominantly Caucasian populations, in North America and Europe, and populations in Japan and China (Jorm, 1991). The consistency of findings suggests that AD is found among all geographic areas and population groups. Differences in prevalence from exposure to different risk factors, if they can ultimately be found, are too small to be detected consistently in population surveys.
The onset of severe cognitive impairment in the ECA study is shown in Figure 5.6. In the ECA study, AD was not separated out from other forms of severe cognitive impairment. The operational measure was the Mini-mental Status Exam. The Mini-mental Status Exam is used as a screening tool for dementia, but it also identifies those with cognitive impairments from other causes, such as delirium, substance-induced hallucinosis, and mental retardation. The at-risk population for onset consists of those with a Mini-mental score of 17 or more, and, among these, the new cases are those whose score at the second wave was 17 or less. The respondents' ability to recall the age of onset was limited because of their cognitive impairment. Hence it was not possible to inquire about the age of occurrence of the first prodromal sign or symptom, for those with cognitive impairment at the second wave, and therefore the figure shows only one distribution. Twenty percent of the onsets occurred before the age of 48, and 50 percent occurred before the age of 68.
GAPS IN OUR KNOWLEDGE
Age-Specific Prevalence in Children
The most important limitation of the ECA data presented above for the illustrative mental disorders is the truncation at age 18. Although the bias of censorship is avoided, there may be significant numbers ofjsible misuse of psychotropic medications has led to regulations in the Omnibus Budget and Reconciliation Act of 1987 intended to thwart misuse. This large gap in the clinical literature has obvious implications for the treatment of those with dementia, andreleasing factor (CRF) (Coy]to maintain minimal personal hygiene; largely incoherent or mute.